Even if the repairs had gone as planned, switching to the backup formatter involves some risk.

The device has been exposed to the same cyclic heat stress that may have caused the failure of the primary side A instrument. Switching to side B would leave Hubble with no backup should that system fail, too.

But the risk would be temporary, as engineers are beginning tests now on a duplicate data formatter, including both a primary and backup system, that has been stored on Earth since Hubble's 1990 launch.